In this document, a transponder is understood to mean an electronic device, which is arranged to communicate with a transceiver when being in a communication range of said transceiver. In the present context, the communication range of the transceiver is understood to mean an alternating electromagnetic field supplied by the transceiver to energize and/or activate the transponder and/or transfer data to the transponder.
The transponder, such as the well-known RFID-tag (“Radio Frequency Identification Tag”) is usually formed as a chip—an integrated circuit (IC)—with an antenna and optional housing material. The antenna is the physical interface for the radio waves to be received and transmitted. The construction of the antenna varies in dependence on the transponder itself and the frequency it operates on. Low-frequency antennas are often made of wire coils, whereas high-frequency antennas are usually made of etched coils or printed with conducting inks. Depending on the application, the transponder may have a housing of glass, or epoxy, or it may be a label, a card, etc. In principle, the transponder may be passive, battery-assisted, or active.
If the transponder is passive, it gets all its power from a signal sent by the transceiver, for example, a reading/writing unit for RFID-tags, also called “reader”. The transponder, using this radio wave to carry the data, is also able to convert it into power. This means that a passive transponder is only powered when it is within the communication range of the transceiver.
In contrast, active transponders have their own power source and may have longer ranges and larger memories than passive tags. The transponder communicates with the transceiver, using what is called the “air interface”. This is a specification for wireless communication between the transceiver and the transponder and includes the frequency of the carrier, the bit data rate, the method of encoding and any other parameters that may be needed. For example, ISO 18000 is the standard for the air interface for item management.
Communication between the transponder and the transceiver can take place in different ways. Some conventional RFID-transponders are designed to operate in an RTF-mode (“Reader Talk First Mode”) or in a TTF-mode (“Transponder Talk First Mode”).
A transponder operating in the TTF-mode announces itself to the transceiver by transmitting an identification (ID) when it detects the presence of the transceiver. In the case of a passive transponder, the transponder talks as soon as it gets power. In other words, a passive transponder is detected by the fact that the transponder powers up when it enters the transceivers communication range.
An active transponder may talk all the time. In the TTF-mode, the active transponder could thus become active on its own, without having to be interrogated for this purpose by the transceiver.
In the RTF-mode, the transponder just waits until it receives a request from the transceiver. This means that, even though the transponder may be activated, it does not talk until it receives a request. In the RTF-mode, data can be written to or read from the transponder by the transceiver.
Since transponders that merely work in the TTF-mode cannot be switched to a reading/writing mode, transponders of this type do not have additional features such as direct offline data storage on the transponder. To overcome this drawback, transponders have been introduced that can be switched from a TTF-mode to an RTF-mode by means of a switching command supplied by the transceiver. Such “switchable” transponders are known as state of the art by the name of “PCF 7930” (a transponder produced by Philips). A state-of-the-art communication system using the PCF 7930 transponder will be elucidated hereinafter, wherein:
FIG. 1 is a block circuit diagram of a communication system according to the state of the art.
FIG. 2 is a flow chart of a communication between a transponder and a transceiver according to the state of the art.
In FIG. 1, the communication system comprises a transponder 1′ of the PCF7930 type. The transponder 1′ comprises a resonant circuit 2 and an integrated circuit (IC) 1a′. Two functions are integrated on the IC 1a′ of the transponder 1′. The first function is an Electrically Erasable Programmable Read-Only Memory (EEPROM) 3. The second function is a contactless interface (CIF) 4′ for power supply, clock recovery and data handling.
The transponder 1′ can be switched between a TTF and an RTF-mode by means of a transceiver 5. The transceiver 5 comprises a data coder 6 and a data decoder 7 as well as a modulator 8 and a demodulator 9 for data transmission. The transceiver 5 generates an RF-field with a predefined frequency, e.g. 125 kHz. The transceiver 5 is arranged to supply a clock signal c and output data d received from the transponder 1′ to a computer 10 and to receive input data d′ from the computer 10.
FIG. 2 shows the transponder 1′ by default in the TTF-mode upon its activation. The TTF-mode is activated when the transponder 1′ senses the magnetic field of the transceiver 5. The point of time when the transponder 1′ is activated or enters the communication range of the transceiver 5 is indicated by BEG in FIG. 2. The direction of the increase of time t is indicated by means of an arrow in FIG. 2. After its activation, the transponder 1′ cyclically sends a number of data blocks, e.g. BL1, BL2, BL3, within a TTF timeframe TF1 to the transceiver 5. When the last data block BL3 is transmitted, a short switching timeframe TF2 with no data transfer is given before the transmission of the first data block BL1 starts again. In the switching timeframe TF2, the transponder 1′ can be switched to an RTF-mode by means of a switching pulse given by the transceiver 5. The switching pulse may also be a switching command. In the RTF-mode, data can be written into the EEPROM of the transponder 1′.
The transceiver 5 recognizes the switching timeframe TF2 due to the fact that no data are received during this time. If no switching pulse is given in the switching timeframe TF2, the transponder 1′ remains in the TTF-mode and continues transmitting its data to the transceiver 5.
It is a drawback of the above-mentioned embodiment that initialization of the transponder takes a long time. Initialization of the transponder means that an initial condition is established, which is necessary for further use of the transponder, e.g. initial programming of the transponder or setting or activating a password for memory access, etc. Initialization is usually done by storing initial data to the transponder, necessary for its further use, such as a transponder ID, etc. Since the transponder is by default in the TTF-mode, one has to wait for the switching timeframe before the initial data can be stored to the transponder.
In situations in which a manufacturer or a retailer intends to initialize millions of transponders, the additional waiting time for the switch window becomes a substantial cost factor.